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how to design a better outdoor fire-pit?  RSS feed

 
Davis Tyler
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So I have built a series of outdoor fire-pits at the various houses I have lived at over the years. From a simple ring of rocks, to a free-stacked masonry chimney. They all worked OK, but the smoke always swirls around in the breeze and gasses the people enjoying the fire. And 90% of the heat goes right up into the air, so your backside is still freezing even when you're right next to the fire.

So I've read about the rocket stoves and rocket mass heaters and I understand the main advantages of complete combustion and trapping and releasing heat. But I'm not sure a RMH is the right technology for an outdoor fire-pit. The "entertainment factor" goes way down by not being able to see the flames dancing above a few pieces of cord-wood. And the cob benches I have seen are great for heating a cabin, but I really don't need a ton of mass to dampen the thermal rise and fall time. I want to light the fire after dinner and have it pumping out heat when it gets dark. Figure 3-4 hours for guests to enjoy the "bonfire" then let it burn out and go to bed. No need for hours of residual heat, though that wouldn't necessarily be a problem.

I'm picturing a full circle of masonry benches, with the fire pit at 12 o'clock, and walkway openings at 4 o'clock and 8 o'clock. So the heat radiates outward to warm the guests and also flows through the benches to warm their asses.

So what's the appropriate technology to build this thing? A RMH with split duct system through cob benches? Rumford fireplace with some sort of exhaust/duct system through the benches? Does a Rumford smoke out in swirling breezes?

Open to any suggestions. It doesn't have to be the most thermally efficient wood-burner, but it's important to entertain the guests while keeping the smoke out of the eyes. Also to create a radiant warm zone through the closed circle of benches.
 
Hans Quistorff
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What I suggest is a Rumford at 12 o'clock and openings at 2 and 10. Then a circle bench with a wall behind it. The bench and wall could be wood which would be warmed by the radiant heat from the Rumford. The round shape wold tend to make the variable breezes always enter next to the fire and not cool the radiant heated seating area.

My experience is with a natural native American fire circle. My grandfather obtained a beach front property with an 8' high by 8' wide by 4' thick granite rock about 8' from the bank. When a bulkhead and cabin was built behind it the foundation of the cabin went on the accumulation of clam shells from generations of use. We could sit with our backs to the bulkhead and not have cold air cool our backs. The prevailing wind was from behind the rock but it had a chimney effect that carried the smoke over our heads. On calm nights there would be variable breezes caused by the temperature differential between the land and the water. The smoke seldom came back on us but it would sometimes go off to one side or the other.
 
Glenn Herbert
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This has been done successfully. See this thread for discussion, including Matt Walker's setup at the 3/14 zone, followed by Erica Wisner's detailed discussion of what works and what doesn't.
http://www.permies.com/t/11556/wood-burning-stoves/Outdoor-rocket-mass-heater-concerns

There is also Matt's outdoor RMH fire ring from the 2014 innovators event at Wheaton Labs (fourth post in this thread):
www.permies.com/forums/posts/list/120/30517
 
Davis Tyler
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thanks for the suggestions and links


Glad to hear the Rumford will work outdoors? Do I need to purchase the pricy pre-cast "throat" or can I fashion something myself out of firebrick and refractory cement?

I'm interested in why you would choose wood for the benches. Wouldn't some sort of masonry be more durable in the long-term? I see the discussion in the other thread about cob not withstanding the elements without a roof. I'm hoping to avoid building a roof so the fire pit and benches would be exposed to the open starts
 
Glenn Herbert
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I noticed this old thread and thought a reply here would be in order.

In the case of a Rumford fireplace, a wood bench makes much more sense than any masonry because masonry would take hours to soak up radiated heat from 5-10' away, while wood could warm quicker and not suck heat from behind people while they are sitting on it blocking the radiant heat.

I expect if you are skilled enough to build the correct throat form for a Rumford fireplace, there would be no reason to buy one.

For an RMH-heated bench, if a roof was not desired, I would use fairly thin brick or other masonry with stone or concrete topping slabs, probably insulated on bottom and back of the channel, so the warmth would soak through within an hour or so. It would not stay warm for more than a couple of hours after the fire was out, but that would not matter for this case.
 
Davis Tyler
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Interestingly I no longer live in the house I was living in when I made the original post, but I'm still on the quest to build the Ultimate Outdoor Firepit(TM) at my new house!

I built a poor man's Rumford at my parent's lake house.  Dry-stacked landscape pavers with a wide and shallow firebox.  I wasn't able to approximate the Rumford throat shape.  Performance is OK, definitely deflects some of the smoke, but still doesn't throw a ton of radiant heat until you get within 3-4 feet.

I have a Home Depot steel chiminea at my current house, but the sides are open 360 degrees, with just screen spark arrestors. This break in the chimney disrupts the chimney effect and sometimes smoke swirls.

I'm coming around to the idea of ditching the masonry bench as suggested above.  Cold masonry is uncomfortable to sit on, and warming it would take too long for a firepit that will be used for 2-4 hours, then allowed to cool.  Lawn chairs and benches can be configured for the size of the party, then moved out of the way to free up walking path when not in use.

So I'm planning/designing the Ultimate Outdoor Firepit(TM)

Requirements:
DIY cost under $200, basic hand tools, no specialty tools required
fit within a volume no greater than 4'x4'x4'
weather resistant 365 days a year in New England (mostly masonry; minimize metal components)
no steel barrel; they look too ugly, even outdoors
10 year service life
Quick startup (20-30 minutes)
allow fire viewing (open combustion or glass door)
supports some minimal outdoor cooking (smores, weenie roast)
primarily radiant heat, radiating around the full circumference of the heater
burn a mixture of deadfall branches, split cordwood, waste 2x4 and pallets scraps, etc.
no smoke swirling in people's eyes
maximum efficiency is not critical, but I think if I minimize the smoke generation I will get enough efficiency for "free"
No mass/storage necessary (when I'm drunk enough to go indoors and sleep, I want the fire safely out)

So I'm thinking along the lines of a 6" Peter van DeBerg batch box with a woodstove glass door, all housed in a cinder-block tower with a two-chamber bell.  Would a 4'x4'x4' tower be enough surface area to slow down the exhaust gases and capture the heat?
 
Devin Lavign
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Something I have found to be helpful to keep the issue of cold backs away is multiple fires.

When I have large groups I make a central fire and a ring of smaller fires 4-6 ft around that. This produces a large warm area that keeps folks warm on all sides. It also increases your ability to look out past the fire, as it makes a wider light ring, and the larger heat area tends to make a larger column of hot air rising reducing smoke in the eyes. 
 
Davis Tyler
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Devin Lavign wrote:Something I have found to be helpful to keep the issue of cold backs away is multiple fires.

When I have large groups I make a central fire and a ring of smaller fires 4-6 ft around that. This produces a large warm area that keeps folks warm on all sides. It also increases your ability to look out past the fire, as it makes a wider light ring, and the larger heat area tends to make a larger column of hot air rising reducing smoke in the eyes. 


that's not a bad idea.  Do you build fires in a ring on the ground, or do you have some sort of burning appliance/raised fire pit?  My patio is about 30'x20' with landscape pavers and it is also a major thru walkway between the house and garden so I don't want too many obstructions
 
Devin Lavign
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Davis Tyler wrote:
Devin Lavign wrote:Something I have found to be helpful to keep the issue of cold backs away is multiple fires.

When I have large groups I make a central fire and a ring of smaller fires 4-6 ft around that. This produces a large warm area that keeps folks warm on all sides. It also increases your ability to look out past the fire, as it makes a wider light ring, and the larger heat area tends to make a larger column of hot air rising reducing smoke in the eyes. 


that's not a bad idea.  Do you build fires in a ring on the ground, or do you have some sort of burning appliance/raised fire pit?  My patio is about 30'x20' with landscape pavers and it is also a major thru walkway between the house and garden so I don't want too many obstructions


I have always done it on the ground, but that is due to being temporary. During parties/festivals or camping for extended periods with large groups. But your looking at a more permanent set up so you might want to adapt the idea a bit for more permanence.

Something I have done many times, is use Swedish torches for the outer ring. Since they are low/no maintenance fires and you can premake a bunch of them to use when needed and lite new ones if the first ones start to die down. If not familiar they are basically log rounds split into segments and the fire is allowed to burn inside them.


How many fires you need to circle your central fire depends on how big that central fire is and how many people need to be kept warm. Using Swedish torches can help you figure out best placement and how many you need.
 
Glenn Herbert
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If you wanted to use Swedish torches in a temporary setup on a stone patio or other place that would be marred by coals and fire on the ground, you could put a trash can lid or the like under each one, with four or so bricks underneath the edges for stability. This would be both safe and neat containment, and also help people avoid accidentally walking too near the fire.
 
Davis Tyler
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I've always wanted to try one of those Swedish torches - are those cuts just a chainsaw kerf?  The "ring of fire" might be a bit dangerous with all the neighborhood kids running around the party
 
Glenn Herbert
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In the simplest form, yes, just a couple of crossing chainsaw kerfs. They have also been made by boring a hole down the center, with an intersecting horizontal hole near the bottom, making a small L-tube configuration.
 
Alan Loy
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When considering chairs, I suggest a high back and an insulated cover such as sheep skin or a customised blanket.  They would help keep the back warm from the inevitable draft.
 
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